Embryo Donation as a Clinical Model: Recipient Body Mass Index and Offspring Sex Distribution After Assisted Reproduction

Background/Objectives: Embryo donation/adoption separates the recipient environment from the genetic origin of the embryo and provides a clinical model for studying offspring sex distribution after assisted reproduction. We evaluated whether recipient body mass index (BMI) was associated with sex distribution among live-birth events after frozen embryo transfer involving embryo donation/adoption. Methods: This retrospective cohort study included frozen embryo transfer cycles involving embryo donation/adoption performed between January 2018 and December 2024 at a tertiary fertility center. All embryos were generated by intracytoplasmic sperm injection, cultured to the day-5 blastocyst stage, transferred singly, and not subjected to preimplantation genetic testing. The analytical cohort comprised 104 independent live-birth transfer events representing 104 unique recipients and 104 unique embryo–donor couples. Recipient BMI was evaluated across quartiles and in an exploratory adjusted logistic regression model including female embryo donor BMI and age, endometrial thickness, serum progesterone concentration, and top-quality embryo status. Results: The cohort included 51 male and 53 female live-birth events. Recipient BMI was higher in female than male live-birth events (25.6 ± 4.3 versus 23.5 ± 4.2 kg/m2; p = 0.007). In a descriptive quartile analysis, male live-birth event proportions were 69.2% in the lowest BMI quartile and 34.6% in the highest quartile, with corresponding female proportions of 30.8% and 65.4% (p for trend = 0.007). In the primary adjusted model, higher recipient BMI was associated with lower odds of a male live-birth event (adjusted OR 0.88 per 1 kg/m2; 95% CI 0.80–0.98; p = 0.017). Conclusions: Recipient BMI was associated with sex distribution among live-birth events after embryo donation/adoption. The observed pattern is compatible with a recipient-side post-fertilization association but does not establish a selection mechanism, causality, or the developmental stage at which the association arose.